1. Field of the Invention
This invention relates to a method of varying the focal length of a zoom lens, and more particularly to zoom lenses having two zooming methods for first and second variations of the focal length suited to still cameras, cinematic cameras or video cameras. Still more particularly, it relates to zoom lenses in which the first variation of the focal length is carried out by the usual method, and the second variation of the focal length is carried out by a different method from that of the first variation to obtain a longer value of the focal length than when in the first variation.
2. Description of the Prior Art
In the past, there have been proposed many methods of extending the range of variation of the focal length of a zoom lens. For example, in Japanese Laid-Open Patent Application No. SHO 57-42010, a rear converter lens unit is inserted into a space between the zoom lens and the image plane to increase the focal length of the entire system, or the so-called rear converter method is used. Another Japanese Laid-Open Patent Application No. 55-32046 proposes the afocal converter method for increasing or decreasing the focal length of the entire system by attaching a lens unit of infinite focal length to the front of a zoom lens.
The rear converter method is advantageous to take the telephoto ratio at relatively small values, but gives rise to an increase in the complexity of structure and the size of the combined system because the rear converter must be well corrected for all aberrations in itself. The afocal converter method, on the other hand, though being easily amenable both to decrease and increase the focal lengths of the zooming range, tends greatly to increase the diameter of the lens unit.
Another or common disadvantage of these two methods is that besides the zoom lens as the basic unit, the attachment lens must always be at hand and, therefore, snap-shooting and portability are not very good.
The other method of extending the range is by increasing the zoom ratio. For example, the total zooming movement is increased, or the refractive power of the zoom unit is strengthened, or the number of zoom units is increased. Any of these methods has, however, a tendency of increasing the total length of the lens, producing large aberrations over the zooming range, or making it difficult to maintain aberration corrections stable throughout the extended range.
It has already been known in the art to change the loci of movement of the zoom units for the purpose of extending the range in U.S. Pat. No. 4,256,371. In this zoom lens, however, the front lens unit remains stationary in that part of the range which may be regarded as basic, or standard.